参数资料
型号: PIC16CE625T-30/SS
厂商: Microchip Technology
文件页数: 43/100页
文件大小: 0K
描述: IC MCU OTP 2KX14 EE COMP 20SSOP
标准包装: 1,600
系列: PIC® 16C
核心处理器: PIC
芯体尺寸: 8-位
速度: 30MHz
外围设备: 欠压检测/复位,POR,WDT
输入/输出数: 13
程序存储器容量: 3.5KB(2K x 14)
程序存储器类型: OTP
EEPROM 大小: 128 x 8
RAM 容量: 128 x 8
电压 - 电源 (Vcc/Vdd): 3 V ~ 5.5 V
振荡器型: 外部
工作温度: 0°C ~ 70°C
封装/外壳: 20-SSOP(0.209",5.30mm 宽)
包装: 带卷 (TR)
其它名称: PIC16CE625T30/SS
1999 Microchip Technology Inc.
DS40182C-page 37
PIC16CE62X
7.2
Using Timer0 with External Clock
When an external clock input is used for Timer0, it must
meet
certain
requirements.
The
external
clock
requirement is due to internal phase clock (TOSC)
synchronization. Also, there is a delay in the actual
incrementing of Timer0 after synchronization.
7.2.1
EXTERNAL CLOCK SYNCHRONIZATION
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of
T0CKI
with
the
internal
phase
clocks
is
accomplished by sampling the prescaler output on the
Q2 and Q4 cycles of the internal phase clocks
(Figure 7-5). Therefore, it is necessary for T0CKI to be
high for at least 2TOSC (and a small RC delay of 20 ns)
and low for at least 2TOSC (and a small RC delay of
20 ns). Refer to the electrical specification of the
desired device.
When a prescaler is used, the external clock input is
divided by the asynchronous ripple-counter type
prescaler so that the prescaler output is symmetrical.
For
the
external
clock
to
meet
the
sampling
requirement, the ripple-counter must be taken into
account. Therefore, it is necessary for T0CKI to have a
period of at least 4TOSC (and a small RC delay of 40 ns)
divided by the prescaler value. The only requirement on
T0CKI high and low time is that they do not violate the
minimum pulse width requirement of 10 ns. Refer to
parameters 40, 41 and 42 in the electrical specification
of the desired device.
7.2.2
TIMER0 INCREMENT DELAY
Since the prescaler output is synchronized with the
internal clocks, there is a small delay from the time the
external clock edge occurs to the time the TMR0 is
actually incremented. Figure 7-5 shows the delay from
the external clock edge to the timer incrementing.
FIGURE 7-5:
TIMER0 TIMING WITH EXTERNAL CLOCK
Q1
Q2
Q3 Q4
Q1
Q2
Q3 Q4
Q1
Q2
Q3 Q4
Q1
Q2
Q3 Q4
External Clock Input or
Prescaler output (2)
External Clock/Prescaler
Output after sampling
Increment Timer0 (Q4)
Timer0
T0
T0 + 1
T0 + 2
Small pulse
misses sampling
Note 1: Delay from clock input change to Timer0 increment is 3TOSC to 7TOSC. (Duration of Q = TOSC).
Therefore, the error in measuring the interval between two edges on Timer0 input = ±4TOSC max.
2: External clock if no prescaler selected; prescaler output otherwise.
3: The arrows indicate the points in time where sampling occurs.
(3)
(1)
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